One of article surveillance systems utilized for the prevention of shoplifting in supermarkets, etc. is an article surveillance system using magnetostrictive materials. The article surveillance system of this type is proposed, for instance, by U.S. Pat. No. 4,510,489. This article surveillance system comprises a marker secured to an article, etc., and a gate for detecting the marker passing therethrough by a receiver comprising one transmitter and two receiving circuits.
The marker is composed of a resonator having soft magnetic properties, and a bias material having semi-hard magnetic properties and placed adjacent to the resonator. Generally, amorphous alloys are used for the resonator, while crystalline materials are used for the bias material. When the bias material adjacent to the resonator is magnetized, the resonator is activated, whereby the marker is activated. On the other hand, when the bias material is demagnetized, the resonator is deactivated, whereby the marker is deactivated. A gate disposed at an exit detects an activated resonator, so that only merchandise that has not been properly accounted for can be detected.
A transmitter and a receiver are placed in the gate at adjacent positions, and the transmitter repeatedly emits a weak AC magnetic field of a particular radio frequency at a certain interval. The receiver is set to operate while the transmitter does not emit the AC magnetic field.
The active resonator resonates when receiving the above AC magnetic field of a particular frequency from the transmitter, thereby emitting a signal. When the transmitter stops generating the AC magnetic field, a signal emitted from this resonator by its resonance is attenuated exponentially. This exponential attenuation characteristic is determined by materials used for the resonator.
Two receiving circuits in the gate detect a signal emitted from the resonator during the idle period of the transmitter with a time lag. This time lag is determined by the distance between the two receiving circuits and the moving speed of the marker. The attenuation characteristics of the signal are determined from the intensity and time lag of these two signals in the gate. When the attenuation characteristics of a particular signal are identical with those measured in advance on the resonator, alarm is generated. Because the signal to be detected can be differentiated from those generated by other articles than the resonator (signals having different attenuation characteristics), this system can advantageously avoid malfunction at the gate.
Required as basic characteristics for the resonator used in the above marker are that a large signal output is generated from the transmitter in an active state by an AC magnetic field, and that the signal has a small attenuation speed.
Used in resonators requiring these magnetic properties are amorphous alloys as described above. The amorphous alloy is usually produced by a liquid-quenching method such as a single roll method in a ribbon form, which is cut to a required shape. In most cases, an amorphous alloy ribbon produced by the liquid-quenching method is heat-treated in a magnetic field to improve magnetic properties and then used for a resonator.
As a method for improving properties necessary for the resonator for use in a marker in an electronic article surveillance system, that is, the intensity and attenuation time of a signal output generated by an AC magnetic field, for instance, U.S. Pat. No. 6,011,475 discloses a heat treatment of an amorphous alloy ribbon in a magnetic field having a predetermined angle to a surface of the amorphous alloy ribbon.
Though the above heat treatment in an angled magnetic field increases an output signal of a resonator, it is increasingly required that the resonator has higher output characteristics, because the output characteristics of the resonator directly affect the sensitivity of an article surveillance system comprising the resonator.